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hours of usable sunlight per year

Google Project Sunroof can give a general idea of how many hours of good sunlight a given roof-mounted solar setup will receive.

maximum and minimum day length

Check timeanddate.com's Sunrise and Sunset Calculator for day length on the following days:

calculation

solar panel's rated power output W P
hours of usable sunlight per year hr Syr
average hours of usable sunlight per day 5.475851494907458 hr Sd = Syr / 365.25
longest day length hr Dl
shortest day length hr Ds
average day length 12.25 hr Da = avg(Dl, Ds)
average solar effeciency guess-timate[0] 45 % E = Sd / Da
longest day solar power output[1] 2593 Whr = P * E * Dl
shortest day solar power output[1] 1788 Whr = P * E * Ds
[0] The average solar effeciency is over the course of the whole year. Therefore, it's probably optimistic in the worst case and pessimistic in the best case. [1] Temperature should also be taken into consideration.